This invention relates to a process for forming printed wiring, more particularly to a process for forming printed wiring on an insulating substrate for producing easily printed wiring boards without including a printing procedure, e.g. for masking, but using an electroless deposition procedure.
Heretofore, there have been proposed various processes for forming printed wiring on insulating substrates for producing printed wiring boards by using electroless deposition procedures. But many of these processes inevitably employ printing procedures therein. For example, U.S. Pat. Nos. 3,619,285 and 3,562,038 propose processes for the deposition of electroless metal on selected areas of a substrate for forming circuits which involve printing a substance which makes the surface easily roughened on the preseribed circuit forming portions or, to the contrary, printing a substance which protects the surface from roughening on portions other than the circuit forming portions in order to make difference in coarseness between the circuit forming portions and the portions other than the circuit forming portions, immersing the thus treated substrate in a catalyst solution, selectively removing the catalyst on the portions other than the circuit forming portions the surface of which are not so roughened with an acid or an oxidizing agent, and effecting electroless deposition. According to these processes, only the circuit forming portions are roughened. For example, an undercoat resin layer (or an adhesive layer) is formed on the whole surface of a substrate, an ink containing an epoxy resin or the like which hardly suffers from roughening by a roughening solution as compared with the undercoat resin layer is coated on portions other than the circuit forming portions by means of printing, and the resulting substrate is immersed in a roughening solution in order to substantially roughen the exposed portions of the undercoat resin layer corresponding to the circuit forming portions. But the disadvantages of these processes are the use of a printing procedure and the use of a masking material to mask the portions other than the circuit forming portions.
Generally speaking, the printing procedure requires an additional elaborate step and an ink used therefor, which makes the production of printed wiring boards complicated and thus undesirable. Further, precision of the resulting pattern is not so good due to the nature of printing technique.
On the other hand, there is disclosed a process for forming printed wiring that does not use printing procedures and instead was ultraviolet light. This process comprises forming a photosensitive layer containing a substance which can function as catalyst after decomposed by light or a photosensitive substance including a catalyst therein on an insulating substrate, exposing circuit forming portions to ultraviolet light to decompose or sensitize the layer, selectively removing nondecomposed layer or nonsensitized layer at portions other than the circuit forming portions, and effecting electroless deposition (e.g. J. Electrochem. Soc. vol. 121, No. 1, pages 67-69 (1974)). Although this process requires no printing procedure, there are some technical problems in this process in that in the case of producing a printed circuit board having one or more through holes it is very difficult to coat the photosensitive layer uniformly in the through holes and to sensitize to the light, and the like.
The present inventors have studied to overcome these disadvantages mentioned above and to provide a simple process for forming printed wiring on an insulating substrate and found that if a photo-setting substance is present in the undercoat resin layer, the undercoat resin layer can be cured by exposing it to light and it cannot be roughened substantially even if it is immersed in a roughening solution. By applying such properties, it was found that when only portions which were not to be roughened, i.e. portions other than the circuit forming portions, were exposed to light, particularly actinic light, and then immersed in a roughening solution, portions to be roughened and portions not to be roughened were produced desirably on the same resin surface without using a printing procedure. In this manner the inventor accomplished the present invention.